Have master-side steps respect the timeout when removing files and directories

This addresses the issue described at <http://trac.buildbot.net/ticket/3055>.
`CompositeStepMixin.runRemoteCommand` makes an equivalent call with `self.rc_log`, which is the log that's created within `CompositeStepMixin.addLogForRemoteCommands`. The SCM steps all have the following within `startVC`:

            self.stdio_log = self.addLogForRemoteCommands("stdio")

The end result is that we're still calling making the call on the same log as we were before.